1. Field of the Invention
The present invention relates to a distributed feedback quantum cascade laser element that oscillates a single-mode light.
2. Related Background Art
The quantum cascade laser element is a monopolar type element that includes a laminate including an active layer formed by laminating emitting layers and injection layers alternately and can generate, in the active layer, light by electron transition between subbands in a quantum well structure. Quantum cascade laser elements are capable of realizing a high-efficiency and high-output operation by using alternately cascade-connected emitting layers and injection layers as active layers, and thus have received attention as sophisticated semiconductor light sources covering the mid-infrared region to the teraheltz (THz) region.
In the quantum cascade laser element, when voltage is applied to the laminate, electrons transit from an upper level to a lower level in a quantum level structure of the light emitting layer, and light with a wavelength according to a difference in energy between the levels is generated at the time of electron transition. The electrons that have transited to the lower level of the emitting layer move to the upper level of a next emitting layer through the adjoining injection layer, and similarly in the emitting layer as well, transit from the upper level to the lower level, and light with a wavelength according to a difference in energy between the levels is generated at the time of electron transition. In this manner, light is generated in each emitting layer.
Meanwhile, in the field of spectroscopic analysis aimed at environmental measurement, as a light source for this, a distributed feedback (DFB) quantum cascade laser element that is capable of continuous wave (CW) oscillation of a single-mode light at room temperature or a temperature in proximity thereof has been expected.
As a DFB quantum cascade laser element capable of CW oscillation of a single-mode light, described in Japanese Published Unexamined Patent Application No. H10-321951 is a top-grating type element including a stripe-structured laminate, having an active layer and cladding layers formed on both sides thereof, formed on a major surface of a substrate and a diffraction grating formed on the laminate.
Moreover, as a DFB quantum cascade laser element capable of CW oscillation of a single-mode light, also known is a buried-grating type element including a stripe-structured laminate, having an active layer and cladding layers formed on both sides thereof, formed on a major surface of a substrate and a diffraction grating formed between the active layer and cladding layer.